fewer/less+adjective+noun

1. a. This city has less beautiful bridges than Paris. b. This city has fewer beautiful bridges than Paris.
2. a. I have less difficult problems than you. b. I have fewer difficult problems than you.
3. a. They drink less fresh water than the city people.

Question 1 : I think these are all correct. But they have different meaning because 'less' is adverb and modifies 'adjective', 'fewer' is adjective and modifies 'noun'.
Therefore, in 1(a) and 2(a) sentences, though 'bridges' and 'problems' are countable nouns, 'less' modifies 'beautiful' and 'difficult'.
In 1(b) and 2(b), 'fewer' modifies 'bridges' and 'problems' Am I right?

Question 2 : As 'less' modifies 'adjective' before 'noun', 'noun' after 'adjective' can be both 'countable noun' and 'noncountable' such as 1(a) 2(a) and 3(a). Am I right?

(1) I think you're right. "This city has less beautiful bridges than Paris (does)". = This city's bridges are not as beautiful as the bridges of Paris are. Here "less" is an adverb. "This city has fewer beautiful bridges than Paris (does)." = Paris has (a few/many) more beautiful bridges than this city has. "Fewer" is an adjective.

All the (a) sentences are ambiguous, and for this reason it's advisable to use 'fewer' before adjectives when it's modifying the following noun. In 1(a) it could be that Paris has less bridges that are beautiful, or it could be that the bridges of Paris are less beautiful. 'Fewer' makes it clear that the latter is meant. (There's no rule that 'less' only quantifies countable nouns - this is repeatedly claimed to be true, but definitely isn't.)

All the (a) sentences are ambiguous, and for this reason it's advisable to use 'fewer' before adjectives when it's modifying the following noun. In 1(a) it could be that Paris has less bridges that are beautiful, or it could be that the bridges of Paris are less beautiful. 'Fewer' makes it clear that the latter is meant. (There's no rule that 'less' only quantifies countable nouns - this is repeatedly claimed to be true, but definitely isn't.)

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Do you mean 'the former'?

I don't know if there is a rule but there should be in my opinion. It sounds better and would remove ambiguities in jullianus' sentences.

All the (a) sentences are ambiguous, and for this reason it's advisable to use 'fewer' before adjectives when it's modifying the following noun. In 1(a) it could be that Paris has less bridges that are beautiful, or it could be that the bridges of Paris are less beautiful. 'Fewer' makes it clear that the latter is meant. (There's no rule that 'less' only quantifies countable nouns - this is repeatedly claimed to be true, but definitely isn't.)

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If I use 'comma', is it possible to avoid these ambiguousness?

1. a. This city has less beautiful bridges than Paris. b. This city has fewer beautiful bridges than Paris.
4. a. This city has less, beautiful bridges than Paris. b. This city has fewer, beautiful bridges than Paris.

In 1(a) sentence, 'less' is 'adverb' and modifies 'beautiful'. In 4(a), 'less' is 'adjective' and modifies 'bridges'
It is more correct to use 4(b) than 1(b) because 'fewer' is adjective. Am I right?